Rotation and differential angle optical sensor with short optical sensing array

ABSTRACT

A sensor may simultaneously sense the angular position of a first rotatable member relative to a frame of reference and the angular position of a second rotatable member relative to the first rotatable member up to a maximum displacement angle. The sensor may include a first and a second disk, each of which have an annular pattern which alternates between two different levels of optical transparency. The first disk may be coupled to the first rotatable member; and the second disk may be coupled to the second rotatable member. A lighting system may direct light toward the first and the second disks with an orientation that causes an annular illumination pattern to be cast upon an optical sensing array which corresponds to a composite of a portion of the first and the second annular patterns and which spans an angle that is less than or equal to the maximum displacement angle. An optical sensing array may provide signals indicative of the annular illumination pattern. The optical sensing array may have a length which is not substantially greater than the width of the annular illumination pattern cast upon the optical sensing array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following U.S. utility applicationsfiled on the same day by the same inventor: “Rotation And DifferentialAngle Optical Sensor With Non-Transition Pattern Sampling,” attorneydocket no. 072209-0023; “Rotation And Differential Angle Optical SensorWith Integral Bearing Races,” attorney docket no. 072209-0024; and“Rotation And Differential Angle Optical Sensor Which Does Not RequireKeyed Installation,” attorney docket no. 072209-0026. The content ofeach of these applications is incorporated herein by reference.

BACKGROUND

1. Technical Field

This disclosure relates to optical sensors, including rotational opticalsensors configured to measure the rotation of a steering wheel in avehicle and the torque that is applied to it.

2. Description of Related Art

Sensors may be used to determine rotational position, such as theposition of a shaft attached to a steering wheel in a vehicle. Thesensors may also determine the torque that is applied to the steeringwheel by measuring the angular displacement between two different shaftswhich are attached to one another through a torsion rod.

Some sensors couple a potentiometer to each rotating shaft to make theseangular measurements. However, potentiometers may fail to operate due tosurface contamination and/or wear related to sliding contacts.

Magnetic sensors may couple a magnet to each rotating shaft and may usea magnetic field detector to make these angular measurements. However,stray magnetic fields and variations in magnet strength may reduce theprecision of the sensors. The components may also be costly.

Optical sensors may couple a disk to each rotating shaft to make tothese angular measurements. Each disk may have an annular pattern whichalternates between different levels of optical transparency. A lightsource and associated optical sensing array may be used to detect aportion of the combined pattern from both disks, which may then betranslated into the angular positions. However, it may be costly tomanufacture and assemble the components, such as the patterns on thedisks, to within a sufficient degree of tolerance so as to achieveneeded accuracy, such as to achieve an accuracy of ±0.02 degrees. Suchsensors may also have to be aligned to a key in the shaft in order tofunction properly. This may add to the cost of the shaft and to theinstallation procedure.

SUMMARY

A sensor may simultaneously sense the angular position of a firstrotatable member relative to a frame of reference and the angularposition of a second rotatable member relative to the first rotatablemember up to a maximum displacement angle. The sensor may include afirst disk, a first coupling, a second disk, a second coupling, anoptical sensing array, a lighting system, and a signal processingsystem.

The first disk may have a first annular pattern which alternates betweentwo different levels of optical transparency.

The first coupling may couple the first disk to the first rotatablemember.

The second disk may have a second annular pattern which alternatesbetween two different levels of optical transparency and may havesubstantially the same radius as the first annular pattern. The seconddisk may be configured such that the first and the second annularpatterns may independently rotate about substantially the same axiswhile remaining substantially adjacent to one another.

The second coupling may couple the second disk to the second rotatablemember.

The optical sensing array may provide signals indicative of an annularillumination pattern which is cast upon the optical sensing array andmay have a length which is not substantially greater than the width ofthe annular illumination pattern cast upon the optical sensing array.

The lighting system may direct light toward the first and the seconddisks with an orientation that causes an annular illumination pattern tobe cast upon the optical sensing array which corresponds to a compositeof a portion of the first and the second annular patterns and whichspans an angle that is less than or equal to the maximum displacementangle.

The signal processing system may generate an output that is indicativeof both of the angular positions based on the signals from the opticalsensing array.

The sensor may include a data storage system containing calibrationdata. The signal processing system may utilize the calibration data whengenerating the output.

The optical sensing array may have a plurality of pixel elements. Eachmay provide a signal indicative of a level of illumination which is castupon the pixel element. The calibration data may include data indicativeof the level of the signal from each pixel element when a known portionof the pixel element is illuminated. The known portion may beapproximately half of the pixel element.

The calibration data may include data indicative of the location of eachpixel element in the optical sensing array.

Each of the optical sensing array elements may provide a signalindicative of the level of illumination which is cast upon the pixelelement. The calibration data may include data indicative of the rate atwhich the signal that is provided by each pixel element changes as afunction of changes in the level of illumination which is cast upon eachpixel element.

The sensor may include a data storage system containing calibration datathat includes data indicative of a location error in each of thetransitions between the two different levels of optical transparency ineach of the annular patterns. The signal processing system may utilizethe calibration data when generating the output.

Each of the disks may be formed by a stamping process.

Each of the disks may have an integral race configured to support ballbearings.

The lighting system may include a light source and a reflecting systemconfigured to focus and collimate light from the light source onto thefirst and the second disks. The light source and the optical sensorarray may be adjacent to the same side of one of the disks and may bemounted to a common circuit board.

These, as well as other components, steps, features, objects, benefits,and advantages, will now become clear from a review of the followingdetailed description of illustrative embodiments, the accompanyingdrawings, and the claims.

BRIEF DESCRIPTION OF DRAWINGS

The drawings disclose illustrative embodiments. They do not set forthall embodiments. Other embodiments may be used in addition or instead.Details that may be apparent or unnecessary may be omitted to save spaceor for more effective illustration. Conversely, some embodiments may bepracticed without all of the details that are disclosed. When the samenumeral appears in different drawings, it refers to the same or likecomponents or steps.

FIG. 1 illustrates components of a rotation and differential angleoptical sensor.

FIG. 2A illustrates a consolidated portion of the annular patterns oftwo of the disks which are illustrated in FIG. 1 and an underlyingoptical sensing array.

FIG. 2B illustrates a binary pattern that is indicative of transitionsin the portion of the consolidated pattern of the two disks illustratedin FIG. 2A.

FIG. 3 is a top view of a rotation and differential angle optical sensorthat may contain the components illustrated in FIG. 1.

FIG. 4 is a sectional view of FIG. 3 along the line 4-4′.

FIG. 5 is an enlarged view of the circled portion of the sectional viewshown in FIG. 4.

FIG. 6 illustrates a first patterned disk from a direction through whichlight may enter.

FIG. 7 illustrates a second patterned disk from a direction throughwhich light may enter.

FIG. 8 illustrates a projected pattern of the first 20 segments of thefirst disk illustrated in FIG. 6 when at a position designated as zero,a projected pattern of the first 20 segments of the second diskillustrated in FIG. 7 when at the zero position, the consolidatedprojection of both disks, and binary values of the consolidatedprojection.

FIG. 9 illustrates the first 20.625 segments of the projected pattern ofthe first disk illustrated in FIG. 6 shifted 0.625 segments clockwise(CW), the projected pattern of the first 20 segments of the second diskillustrated in FIG. 7 when at the zero position, the 20 segments of theconsolidated projection of both disks, and the binary values of theconsolidated projection of both disks.

FIG. 10 illustrates the first 21 segments of the projected pattern ofthe first disk illustrated in FIG. 6 shifted 1 segment CW, the projectedpattern of the first 20 segments of the second disk illustrated in FIG.7 when at the zero position, and the binary values of the consolidatedprojection of both disks.

FIG. 11 illustrates six 20 bit binary vectors. The first vector is aprojected pattern of the first disk illustrated in FIG. 6 when at thezero position, the second vector is a projected pattern of the seconddisk illustrated in FIG. 7 when at the zero position, the third vectoris the compliment of second vector, the fourth vector is the logicalright shift of the first vector, the fifth vector is the logical rightshift of second vector, and the sixth vector is the bitwise product ofthe first, third, fourth and fifth illustrated vectors.

FIG. 12 is a flow diagram of a process that may be implemented by arotation and differential angle optical sensor.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments are now discussed. Other embodiments may beused in addition or instead. Details that may be apparent or unnecessarymay be omitted to save space or for a more effective presentation.Conversely, some embodiments may be practiced without all of the detailsthat are disclosed.

Two patterned disks, a light source, and an optical sensing array may beused. The patterns may lie along circular tracks that alternate betweentwo levels of transparency. One level may be completely opaque; theother level may be completely transparent. The components may bearranged so that an image of a sector of the consolidated patterns inboth tracks is projected onto the optical sensing array. The diskpatterns may be chosen such that the projection resulting on the opticalsensing array is unique for every rotation of the first disk betweenzero and 360 degrees and for every rotation of the second disk relativeto the first disk within a predefined angle. The consolidated projectedpatterns of the disks may allow the positions of both disks to becomputed from the output of a single optical sensing array. Acalibration routine may eliminate the need to manufacture or positionthe components of the sensor with a high degree of precision.

The disks may contain patterns with pattern boundaries nominallyoccurring at integer multiples of 360/N degrees, where N is a positiveinteger. The required tolerance on the boundary separations may be plusor minus 360/4(N) degrees. A smaller N may give a larger minimumboundary separation and a larger tolerance on the boundary position.Sensors may use a small N, such as an N that is less than 1200.

The absolute position sensor may resolve the angular position of onedisk for any rotation between zero and 360 degrees and the angularposition of a second disk relative to the first disk over a range ofzero to D degrees. D may be determined by the pattern design and may belimited to 360 degrees. D may be referred to as the maximum differentialsensing angle.

The disk patterns may not require fine or precise geometry and may beproduced with commercial stamping processes. Large transparent areas inthe annular patterns may be used which are less likely to be obstructedby stray particles. Stamped bearing races may be integrated into thedisks. Alternate methods of making the disks may include photo etchingand molding.

The sensor may be useful in steering systems. Many vehicles may requiresteering angle and applied steering torque information. When a sensor ofone of the types described herein is installed, a first disk may becoupled to the steering shaft and a second disk may be coupled to a handwheel shaft. The hand wheel shaft may be connected to the steering shaftby a torsion rod. Steering angle, measured with respect to the vehicleframe, may be output by determining the rotation of the first disk.Rotating the hand wheel shaft may apply torque to the steering shaft bytwisting the torsion rod. The angular twist of the torsion rod may bethe differential angular rotation between the first and second disks.The applied torque may be calculated from the differential angle (thetwist of the torsion rod) by using Hooke's law and the stiffness (springrate) of the torsion rod. The angular position of the first diskrelative to the second disk may be accurately determined up to a maximumdifferential sensing angle which may equal or exceed the angular span ofthe optical sensing array.

FIG. 1 illustrates components of a rotation and differential angleoptical sensor. The sensor (shown in a top view in FIG. 3 and a crosssection view in FIG. 4) may include two patterned disks, such as a firstdisk 3 and a second disk 4, a light source 1, a signal processing systemand associated data storage system 38, and an optical sensing array 2.

The optical sensing array 2 may include a number of sensing pixelelements, such as 128 or any other number of sensing pixel elements. Thelight source 1, the signal processing system and associated data storagesystem 38, and the optical sensing array 2 may be mounted on a printedcircuit board 6 that may be held in a needed position by a pad 19.

Annular patterns 7 and 8 may alternate between two different levels ofoptical transparency and may be on the disks 3 and 4, respectively. Thepatterns may lie along a circular track of nearly equal diameter on eachdisk. The patterns may be relatively coarse, may not require highprecision, and may be produced by a commercial stamping process.

The disks 3 and 4 may be held so they rotate about a common axis withoutbeing able to translate relative to the optical sensing array 2. Theaxis of disk rotation may pass through the center of each circular trackand may be normal to the patterned surfaces and optical sensing arraysurface.

As shown in the detail view in FIG. 5, mirrors 5 a and 5 b may directlight along a path 17 from the light source 1 and through a portion ofthe patterns on the disks 3 and 4 so that a consolidated projection ofthe two patterns is cast onto the optical sensing array 2. The lightsource 1 may be a surface mounted light emitting diode (LED) or anyother type of light-emitting device. The surface of mirror 5 a may becurved in a manner that focuses light coming from the light source 1onto the mirror 5 b. The surface of mirror 5 b, in turn, may be curvedin a manner that collimates light coming from the mirror 5 a as ittravels to the patterns in the disks 3 and 4. A single mirror, more thantwo mirrors, and/or other types of optics may instead be used to performboth the focusing and collimation functions.

The light source 1 and the optical sensing array 2 may both be mountedon the single printed circuit board 6. This may provide a stable, rigid,accurate, and low cost mounting for these components. By focusing thelight, lower emitted light power may be used without diminishing thelight intensity incident on the optical sensing array 2. Reduced powerconsumption, decreased heat generation, and increased LED life mayresult. The collimating surface of the mirror 5 b may reduce projectedpattern variation resulting from distance variation between the sensingsurface and the patterned disc surface. The design may be implementedwithout the mirrors 5 a or 5 b; however, this may require a differentmounting arrangement for the light source 1.

The disks 3 and 4 may include integral ball bearing races 12 a and 12 b,respectively. The patterned disks 3 and 4 may be produced by stampingsheet metal. The integral ball bearing races 12 a and 12 b may be formedas part of this stamping process. Two additional separate ball bearingraces 13 may also be provided.

Three sets of ball bearings 11 may run in the bearing races 12 a, 12 b,and 13 and may regulate the rotation and movement of both disks. Eachset of ball bearings may have about ten balls that may be kept separatedfrom one another by ball spacers 14. The formed bearing races 12 a, 12b, and 13 may have a central axis coaxial with the central axis of thedisk pattern within the limits of the tool fabrication. Balls rollingalong these races may reduce wear and friction.

The sensor may include a compliant member 18 that may load the racesagainst the balls to minimize any radial or axial motion of the diskswith respect to a lid 34 and a housing 35.

The first disk may be connected to a compliant coupling 15 and thesecond disk may be connected to a compliant coupling 16. The compliantcouplings may be configured to couple the sensor to a first and secondrotatable shaft, respectively. The compliant couplings may preventmisalignment and run-out on the first and second shafts from imposingrun-out on the first and second disks. These coupling features may allowthe sensor to tolerate significant shaft run out without degradingaccuracy.

The patterns 7 and 8 may consist of transparent segments 36 and opaquesegments 37 as shown in FIGS. 6 and 7. The light source 1 and theoptical sensing array 2 may be arranged such that light may pass throughthe transparent segments of the second disk 4, impinge upon the firstdisc 3, pass through the transparent segments of the first disk 3, andimpinge upon the optical sensing array 2. Hence, a pattern ofilluminated and darker areas may be formed on the optical sensing array2. The resulting pattern may be considered binary by assigning ‘1’ toilluminated areas, and a ‘0’ to the darker areas.

Overview of Sensor Operation

FIG. 12 is a flow diagram of a process that may be implemented by arotation and differential angle optical sensor. An overview of thisprocess will now be provided. Details about one or more approaches forimplement each step follow.

The sensor may capture the consolidate pattern image which is cast uponthe optical sensing array 2, as reflected by Capture Image step 101. Theanalog signal from each pixel of the optical sensing array 2 may besensed and stored.

The sensor may then digitize each of the captured analog signals, asreflected by an A/D Conversion step 103.

The location of each transition in the captured pattern may bedetermined, as reflected by a Locate Transition step 105. Calibrationdata may be used to make these determinations, as described in moredetail below. The resulting set of transitions is referred to below asthe function G(x).

Locations may next be determined at which the captured pattern is to besampled, as reflected by a Compute Non-Transition Sampling Locationsstep 107. Each sampling location may be at a location in the pattern atwhich there is no transition. This may eliminate an error that mightotherwise be caused by imprecision in the transition.

The captured pattern may then be sampled at each sampling location, asreflected by a Sample Pattern at Non-Transition Sampling Locations step109. A vector which may have M bits may be defined by the values in thissample where there may be one bit for each sample value.

The angular position of the first disk relative to a frame of referenceand the angular position of the second disk relative to the first disk,both to the nearest segment, may then be determined using a look-uptable (or other means), as reflected by a Lookup Integer Rotations step111.

Greater precision in the measurements may then be obtained bydetermining the precise position of a transition edge on each disk withrespect to a pixel element in the optical sensing array, as reflected bya Find Isolated Boundaries step 113. An “isolated” boundary is aboundary that lies on one of the disks at the detected position, but notthe other.

The amount by which the position of each disk may vary from thepositions of the nearest segment that were previously determined in step111 may be calculated based on the located isolated boundary, asreflected by a Calculate Fractional Rotations step 115. Calibration datamay again be used during this calculation.

The precise positions of each disk may then be outputted, as reflectedby an Output Positions of Both Disks step 117.

The computations and lookups which have been described may be repeatedperiodically and/or on command to determine changes in the position ofone or both of the rotatable members to which the sensor may be coupled.

The conversions, computations, and lookups which have been described maybe performed by a signal processing system. The signal processing systemmay include an analog-to-digital converter, a data processing system,and a memory storage system. The data processing system may includecomputer hardware and software configured to perform the steps andfunctions which have been described herein. The computer software mayinclude one or more algorithms configured to perform these steps andfunctions.

Disk Patterns

A pattern boundary may exist at every position along the pattern trackwhere the pattern transparency changes from opaque to transparent. Achosen pattern boundary on each disk may be assigned as the 0 degreepositions 9 and 10. The direction of angular measurement may be assignedby viewing the disks from the side that light enters. Angular position ron the first disk may be defined as the counter-clockwise (CCW) angle indegrees from the 0 degree position 9. Angular position s on the seconddisk may be defined as the CCW angle in degrees from the 0 degreesposition 10.

Disks fabricated according to the invention may be partitioned into Nnominally equal segments, and the transparency level may be constantover each segment. Hence, the pattern for each disk may be given by avalue of 0 or 1 for each segment i*360/N≦r,s<(i+1)*360/N degrees where iis an integer and 0≦i<N. If F1(u) and F2(u) give the transparency levelsat angle u segments for the first and second disk respectively, the diskpatterns may be given by F1(i+0.5) and F2(i+0.5) for i={0, 1, 2, . . .(N−1)}.

For the specific embodiment shown in FIG. 5 and FIG. 6, N may equal 360.F1(i+0.5) may be given by:

-   -   {0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0,        0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0,        1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0, 1,        1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,        1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1,        1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1,        1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 0,        0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0,        0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0,        0, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0,        1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1,        1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1,        1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1,        1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1,        1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,        1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1,        1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 0,        0, 1, 1}.

F2(i+0.5) may be given by: {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 0, 1,1, 1, 0, 1, 0} (this 20 bit pattern may repeat 17 more times).

Projected Pattern with Disks at Zero Position

At least M segments of each disk may be projected onto the opticalsensing array 2. M may be a positive integer.

FIG. 2A shows an example of the disk patterns positioned over theoptical sensing array and FIG. 2B shows the binary output of the opticalsensing array 2 after signal processing. The 0 degree rotation of thefirst disk may be given by the rotation where the 0 degree position 9projects directly over the CW end of the optical sensing array 2. Thezero degree rotation of the second disk may be given by rotation wherethe 0 degree position 10 projects directly over the CW end of theoptical sensing array.

The M bit pattern on the optical sensing array may be given by (F1(0.5)AND F2(0.5), F1(1.5) AND F2(1.5), . . . F1(M−0.5) AND F2(M−0.5)). Thismay be shown schematically in FIG. 8; diagram 22 shows the projectedpattern of the first disk 3 at the zero position, diagram 21 shows theprojected pattern of the second disk 4 at the zero position, diagram 25shows the consolidated projected pattern of both disks, and diagram 39is the 20 bit binary vector of diagram 25. If the first disk has beenrotated CW by i segments and the second disk has been rotated by jsegments, then the projected pattern may form an M bit dimensionalvector on the optical sensing array and may be given by (F1(i+0.5) ANDF2(j+0.5), F1(i+1.5) AND F2(j+1.5), . . . F1(i+M−1+0.5) ANDF2(j+M−1+0.5)).

FIG. 10 is a schematic representation of the projected pattern with thefirst disk rotated CW one segment (i=1) and the second disk at the zeroposition (j=0). In FIG. 10; diagram 24 shows the projected pattern ofthe first disk 3 rotated 1 segment CW from the zero position, diagram 21shows the projected pattern of the second disk 4 at the zero position,diagram 27 shows the consolidated projected pattern of both disks, anddiagram 41 is the 20 bit binary vector of diagram 27.

Determining Integer Rotations

The angular position of each disk may be determined to integer multiplesof a segment in the following way. Let x be the position measured insegments along the optical sensing array starting at the CW end of thearray as shown in FIG. 2B. Let G(0) be equal to the illumination valueon the optical sensing array at x=0 resulting from the projection ofF1(i) AND F2(j), where i is the position of the first disk over the CW(starting end) of the optical sensing array and where j is the positionof the second disk over the CW (starting end) of the optical sensingarray. Let G(x) be equal to the illumination value on the opticalsensing array resulting from the projection of F1(i+x) AND F2(j+x). Thenthe M bit dimensional vector on the optical sensing array may be givenby (G(0.5), G(1.5), . . . G(M−0.5)).

If 0≦i<N, and i≦j<i+K, then there are N*K unique binary patternscorresponding to the N segment rotations of the first disk with the Ksegment rotations of the second disk. K may be an integer, which may be≧M. K may be called the maximum differential sensing angle and may bedetermined by the pattern construction.

One method of relating the binary vectors of length M to the i and jdisk rotations may be by constructing a lookup table. The (K*i+j)thentry in the table may be made the M bit dimensional vectorcorresponding to the i segment rotation of the first disk with the (i+j)segment rotation of the second disk. In the specific embodiment, Mequals 20. Other values may be used instead.

Selecting Appropriate Value of x_(i) and x_(j)

The disks are not restricted to rotating to angular positions that arean integer number of segments. For example, FIG. 9 shows the first diskrotated 0.625 segments CW from the zero position and the second disk inthe zero position. In this case, the M bit binary vector may beconstructed from illumination levels that are not located in the middleof each segment, because the illumination level in the middle of asegment may be very close to a transition which may lead to an incorrectvalue. Let x_(i) and x_(j) be rotations of the first and second disks,respectively, that are fractions of a segment. In this case, the angularposition of each disk may be expressed as the sum of the integerrotation and the fractional rotation; i+x_(i) for the first disk andj+x_(j) for the second disk. To construct the binary vectors of M bits,illumination levels on the optical sensing array may be sampled atspecific locations away from boundary transitions. These samplinglocations may depend on x_(i) and x_(j). Due to manufacturingimperfections, the pattern boundaries may not be spaced by exact integermultiples of segments. Hence, x_(i) and x_(j) may vary from boundary toboundary.

An algorithm for selecting an appropriate value of x_(i) and x_(j) maybe used. For this method, the variation of x_(i) and x_(j) are eachcontained in an envelope of at most 0.25 segments. To make theselections, map the fractional part f, a member of [0,1), to the unitcircle. The unit circle, c(f), is mapped by c(f)=360*f degrees. Anglesmay be designated to increase from zero in the CCW direction. Then, allc({x_(i)}) lie in an arc of at most 90 degrees, and all c({x_(j)}) liein an arc of at most 90 degrees. It follows that there exists an arc, A,that contains no x_(i) or x_(j), of at least 90 degrees.

Let A₁ be the CW end of A, and A₂ be the CCW end of A, where A is thearc starting at A₁ and continuing CCW to A₂. Take c⁻¹(A₂+45) for x_(i)and c⁻¹(A₁−45) for x_(j). At this point, it may not matter if x_(i) isthe fractional rotation of the first or second disk.

Sampling Location for Fractional Rotations

The values for x_(i) and x_(j) described above may be used to constructthe M bit dimensional vector. Consider the case where one disk has beenrotated CW i+x_(i) segments and the other disk has been rotated CW byj+x_(j) segments with 0≦x_(i), x_(j)<1. In this case, the M bitdimensional vector is as follows:

-   -   if x_(i)≦0.25, and (x_(i)≦x_(j)<x_(i)+0.5 or        x_(j)+0.5≦x_(i)<x_(j)+1) use (G(0.25−x_(i)), G(1.25−x_(i)), . .        . G(M−0.75−x_(i)));    -   if x_(i)≦0.25 and (x_(i)0.5≦x_(j)<x_(i)+1 or        x_(j)<x_(i)<x_(j)+0.5) use (G(0.75−x_(i)), G(1.75−x_(i)), . . .        G(M−0.25−x_(i)));    -   if x_(i)>0.25 and (x_(i)≦x_(j)<x_(i)+0.5 or        x_(j)+0.5≦x_(i)<x_(j)+1) use (G(1.25−x_(i)), G(2.25−x_(i)), . .        . G(M−0.25−x_(i)));    -   if 0.25<x_(i)≦0.75, and (x_(i)+0.5≦x_(j)<x_(i)+1 or        x_(j)<x_(i)<x_(j)+0.5) use (G(0.75−x_(i)), G(1.75−x_(i)), . . .        G(M−0.25−x_(i)));    -   if 0.75<x_(i)<1 and (x_(i)+0.5≦x_(j)<x_(i)+1 or        x_(j)<x_(i)<x_(j)+0.5) use (G(1.75−x_(i)), G(2.25−x_(i)), . . .        G(M+0.75−x_(i))).

This M bit binary vector may now be used to determine the integersegment rotations by using a lookup table. An example of theconstruction of this vector is shown schematically in FIG. 9; diagram 23shows the projected pattern of the first disk 3 rotated 0.625 segment CWfrom the zero position, diagram 21 shows the projected pattern of thesecond disk 4 at the zero position, diagram 26 shows the consolidatedprojected pattern of both disks, and diagram 40 is the 20 bit binaryvector of diagram 26 constructed by using the method given above.Approaches other than a lookup table may be used in addition or instead,such as by performing algorithmic calculations.

Defining Isolated Boundaries

A segment may be a sector of any angle, such as one degree. The size ofa segment may depend on the value chosen for N. An application mayrequire rotations to be resolved within a small fraction of a segment.To describe how fractional disk rotations may be resolved, the term“isolated boundary” is defined as follows. Let P=(p₀, p₁, . . .p_((M−1))) be the M bit dimensional vector which would be projected ontothe optical sensing array as a result of only the first disk rotated aninteger number of segments. Let T=(t₀, t₁, . . . t_((M−1))) be the M bitdimensional vector which would be projected onto the optical sensingarray as a result of only the second disk being rotated an allowedinteger number of segments. If t_(i) and t_((i+1)) have different valuesand p_(i) and p_((i+1)) are both 1, the second disk may be said to havean isolated boundary at i+1. If p_(i) and p_((i+1)) have differentvalues and t_(i) and t_((i+1)) are both 1, the first disk may be said tohave an isolated boundary at i+1.

Locating Isolated Boundaries

Perform a right shift of T and P so that T₁=(t₁, t₂, . . . t_((M−1)), 0)and P₁=(p₁, p₂, . . . p_((M−1)), 0). Define the compliment operator, ˜,on an M dimensional binary vector by (˜T)=(˜t₀, ˜t₁, . . . ˜t_((M−1))).Define the bitwise product of vectors as PT=(p₀*t₀, p₁*t₁, . . .p_((M−1))*t_((M−1))). In algebraic notation, an isolated second diskboundary exists at i+1 segments from zero whenever PP₁(˜T)T₁ orPP₁T(˜T₁) is 1 at bit i. An isolated first disk boundary exists at i+1segments from zero whenever TT₁(˜P)P₁ or TT₁P(˜P₁) is 1 at bit i. Thecase i=19 may be excluded, since bit 20 may be of unknown value. Thepatterns may be designed so that for every allowed integer segmentrotation of the disks, at least one first disk isolated boundary and onesecond disk isolated boundary exists.

FIG. 11 illustrates information that may be used in a determination ofisolated boundaries. Diagram 22 represents the 20 bit binary vector Pfor the first disk at position zero degrees. Diagram 21 is the 20 bitvector T for the second disk at zero degrees. Diagram 30 is thecompliment (˜T) of diagram 21. Diagram 28 is P₁ for P as shown indiagram 22. Diagram 29 is T₁ for T as shown in diagram 21. Diagram 33 is(˜T)T₁PP₁ and has value 1 for bits 11 and 13 indicating that diagram 21has isolated boundaries at 12 and 14 segments from zero. Diagrams 31 and32 indicate all the isolated second disk boundaries when the first andsecond disks are at the zero position.

Determining Fractional Rotations

When a first disk isolated boundary has been identified, the fractionalrotation of the first disk may be given by the actual position of theisolated boundary minus the integer position of the boundary insegments. The fractional rotation of the second disk may be computedsimilarly. One method that may be used to identify the actual positionof a boundary is detailed in the paragraphs below describingcalibration.

Calibration of Optical Sensing Array

Calibration may allow the sensor to correct for anomalies in thegeometry and location of the optical sensing array and the patterneddisks.

The optical sensing array may be calibrated first. Calibration of the128 pixel optical sensing array consists of measuring and storing 3parameters for each pixel in the array. These are: pixel midspan valueq_(i), pixel midpoint location v_(i), and pixel slope m_(i). Thesevalues may be stored in three 128 element arrays in a data storagesystem within the sensor. These parameters may then use for linearinterpolation to calculate the location of transitions. The lightsensitive elements comprising the optical sensing array may includeareas of known boundary that produce an electrical response ranging froma minimum value to maximum value as the light power incident on theirsurface varies from a minimum to a maximum. Several of said lightsensitive elements may be employed in the sensor and their relativeangular locations in the sensor may be calibrated to improve theaccuracy of the sensor.

In one embodiment, the optical sensing array may consist of 128 elements(pixels). The element locations may be calibrated next. The first lightsensor element may be chosen as the zero degree element. The patterneddisks may be rotated CCW in unison, so that an opaque area, followed bya transparent area passes over the zero degree element. The disks may beoriented so that the opaque and transparent areas have greater projectedwidth than the element area. As a result, the zero element response maychange from a minimum to a maximum. The instrument rotating the discsmay be equipped with a high accuracy, high resolution rotationalmeasurement device. The rotation at which the zero element responseequals (maximum−minimum)/2 may be noted. The rotation may be continuedover all the remaining light sensing elements with the rotation angle atwhich each element achieves its mid-span response being recorded in adata storage system within the sensor. These relative locations of thesensing elements from the zero element may be termed the midpoint. Allmidpoint locations, v_(i) and midpoint values q_(i) for each element i,may be recorded in the data storage system in the sensor. For theembodiment described in this specification, the rotating instrument mayhave an accuracy better than 0.0025 degrees and a resolution of at least0.001 degrees.

As noted above, the response of each light sensitive element may varyfrom a minimum value to a maximum value as the projected pattern on theelement varies from entirely dark to entirely illuminated. This responsechange may occur over an angular displacement approximately equal to thearc length spanned by the element under consideration. Deviations fromthis theoretical length may occur for a variety of reasons, includingimperfect parallelism in the light, imperfect parallelism of the diskswith sensing element surfaces, and imperfect alignment of the diskpattern boundaries with sensing element boundaries.

For this reason, a pixel slope calibration may also be recorded in thedata storage system for each sensing element used in the sensor. Definem_(i)=W/(Rmax−Rmin). m_(i) has the units of degrees per elementresponse. Here W is chosen to be the arc subtended by the averageoptical sensor element, Rmax is the element response when the boundarythat transitions from transparent to opaque has been rotated W/2 degreespast the element midpoint, Rmin is the element response when theboundary that transitions from transparent to opaque lies W/2 degreesbefore the element midpoint and we have indexed the elements by index i.The useful resolution of a sensor so constructed may be (elementnoise)(m_(i)) degrees. Nonlinearity of the element response, calibrationerrors, and mechanical noise may further limit the useful sensorresolution.

Disk Calibration

The patterns on each disk may not be perfectly positioned. Calibrationof the disks may consist of measuring and storing the error in the edgelocation of each edge (boundary) on each disk. These values may bestored in two arrays (one for each disk). The array sizes may be basedon the pattern chosen. The following method may be used to compensatefor these inaccuracies.

A boundary may be either a falling boundary or a rising boundary. Afalling boundary may occur at position v_(i)+m_(i)*(pix_(i)−q_(i)),degrees where pix_(i) is the element response of element i, which may beless than q_(i), and the response pix_((i−1)) may be greater thanq_((i−1)). A rising boundary may be analogous. Disk calibration mayallow typical fabrication tolerances without degrading accuracy. Addingcalibration data to the data storage system may increase the accuracy ofthe sensor.

The edge positions may be calibrated by recording the difference, actualposition minus nominal position, as an offset for each pattern boundaryin the first and second disks. The reported rotation angle may bemodified by adding the calibration offset to the rotation angle computedfrom edges assumed to be located at nominal position. The offset usedmay be the offset corresponding to the isolated pattern boundary used tocompute the fractional component of the rotation as described in aprevious paragraph. For a disk of perfect form, all offsets may be zerodegrees. Both disks may be calibrated by rotating them using aninstrument of high angular precision and high resolution. A particularsensing element may be selected and the angle at which the first patternboundary is incident with this element midpoint may be recorded in thedata storage system. The angular rotation at which each successiveboundary becomes incident with the selected element midpoint may also berecorded in the data storage system. If the pattern construction hasperfect form, all the recorded positions differ by an integer multipleof 360/N. Typically, the position of each boundary may occur at anominal rotation +ε where ε is the error term for the boundary position.An ε term may be recorded for each boundary in the data storage system.

These ε terms may be used to improve the accuracy of the sensor whenreporting rotation. Let h be an integer [0,359]. A boundary at a nominallocation of 360 h/N degrees CCW of the zero element midpoint maynominally be rotated 360 h/N degrees CW to be located at the zeroelement midpoint. The actual rotation required to bring the boundary inalignment with the zero element midpoint may be (360 h/N+ε) degrees CW.Each disk may be calibrated in a similar manner.

Simple Pattern

A pattern may have N=360 and M=20. The selected processor may not beable to perform a 7200 entry lookup within the required response time.For this reason, the patterns may be designed to be separable into firstand second disk patterns. The first disk pattern may splice 18 ten bitpatterns with their 10 rotations to produce 180 unique 10 bit patterns.Each bit may be two degrees wide to complete the 360 degree pattern. Thesecond disk pattern may be defined by the ten bit pattern equivalent ofthe hexadecimal value 0x9f for the even bits with ones inserted for theodd bits, producing a 20 bit pattern. This 20 bit pattern may berepeated 18 times to complete the 360 degree pattern. By interleaving1's in the second disk pattern, one of the 180 distinct first diskpatterns is always projected. A 10 bit first word may be constructedfrom the even bits and a 10 bit second word from the odd bits. The wordwith the most number of ones may be the first disk pattern and can befound in a 180 pattern table. The first disk rotation may be an evennumber of segments if the second disk word equals (first disk pattern)AND (a rotation of 0x9f). Otherwise, the first disk rotation may be odd.For even second disk rotations, the first disk word may be the wordderived from the odd bits. The relative rotation of the second disk maybe calculated since the second disk pattern cycles through each of its10 rotations in 20 segments.

The algorithm for determining the position of both disks described abovemay be based on the specific pattern and hardware chosen; otheralgorithms may be devised, depending on pattern selection and thehardware chosen to compute the positions.

Disk patterns may be designed by first considering the sensorrequirements. One disk may be configured to resolve any rotation over afull revolution and a second disk may be configured to resolve a maximumdifferential sensing angle. In the embodiment described, a 20 degreedifferential rotation may be determined.

Mounting requirements may be a consideration. In one embodiment, a shaftof a diameter up to 24.6 mm may pass through the center of the disks.This may limit the minimum diameter of the disks. Selection of theoptical sensing array may constrain the design. The sensor housing maybe large enough to house the optical sensing array. The optical sensingelement(s) in the optical sensing array may limit the spatialresolution. Let e be the nominal spacing of the elements in the array.The pattern radius b may be large enough so that the angular resolution(arctan(e/b)) is satisfactory for the application. Economicconsiderations may constrain the mechanical position variation, for afixed rotation, to a value of z. The pattern radius b may be largeenough to insure that the angular variation resulting from thismechanical variation, arctan(z/b) degrees, does not exceed the allowablesensor output variation. After consideration, a minimum pattern radiusand an optical sensing array may be chosen.

With the optical sensing array and pattern radius determined, the arcspan of the sensing elements, a rotations, may be computed. In oneembodiment, α<<1. If N is the number of pattern segments in a fullrotation, αN may be the number of segments visible in the projectedimage. For a binary pattern, 2^((αN)) binary patterns or less may beprojected onto the sensing element(s). The choice of N may alsodetermine the minimum number of distinct projected patterns required.There may be N integer rotations by a segment corresponding to a fullrotation of the first disk and, for each of those N segment rotations,there may be βN segment rotations corresponding to the allowable senseddifferential rotation of the second disk. To uniquely determine each ofthe rotations, at least βNN distinct projected patterns may be used.Neglecting the complication of rounding to integers, the relation2^((αN))≧βNN may be implied. This may be a strict limit on the minimumsize of N and may not be obtained because 2^((αN)) may over estimate theactual number of patterns that may be designed into the disk.

Define the term ‘weight’ of a pattern to equal the number of ones in aprojected pattern. Let q be the minimum weight of all the binarypatterns projected by the first disk. There may be at least βN combinedprojected patterns for any fixed first disk pattern. Hence, 2^(q)≧βN,where 2 may equal the maximum possible distinct combined patterns forthe first disk pattern with weight q. This may eliminate all patterns oflower weight from the 2^((αN)) estimate. A better estimate for N may begiven by 2*C(M,M/2)≧βNN (where C is the combination function) where Mmay be chosen so that M≦αN. M is the number of bits in the projectedpattern. All binary patterns considered for this invention may haveprojected patterns for the first disk of a least two weights. A diskwhich projects only one weight of patterns may not have N larger than M.

The first and the second annular patterns may be configured such thatthe angular displacement of the second rotatable member relative to thefirst rotatable member angle may be accurately indicated by the outputof the sensor up to a maximum displacement sensing angle, regardless ofhow the first annular pattern is aligned with respect to the secondannular pattern at the time the couplings are coupled to theirrespective rotatable members. The annular patterns illustrates in FIGS.6 and 7 are an example of such annular patterns.

The annular pattern in one of the disks may include or consist ofrepetitions of a sub-pattern. The sub-pattern may have multipletransitions between the different levels of optical transparency. Thesub-pattern may span an angle that is substantially equal to the maximumdisplacement sensing angle. The annular patterns illustrates in FIGS. 6and 7 are an example of such annular patterns.

The preceding discussion pertains to a sensor for detecting angularrotations. The same principles may apply equally to a sensor fordetecting linear displacements. The disks could be replaced withtranslating members and the annular patterns could be laid out in astraight line.

The components, steps, features, objects, benefits and advantages thathave been discussed are merely illustrative. None of them, nor thediscussions relating to them, are intended to limit the scope ofprotection in any way. Numerous other embodiments are also contemplated.These include embodiments that have fewer, additional, and/or differentcomponents, steps, features, objects, benefits and advantages. Thesealso include embodiments in which the components and/or steps arearranged and/or ordered differently.

Unless otherwise stated, all measurements, values, ratings, positions,magnitudes, sizes, and other specifications that are set forth in thisspecification, including in the claims that follow, are approximate, notexact. They are intended to have a reasonable range that is consistentwith the functions to which they relate and with what is customary inthe art to which they pertain.

All articles, patents, patent applications, and other publications whichhave been cited in this disclosure are hereby incorporated herein byreference.

The phrase “maximum displacement angle” refers to the differential anglethat can be sensed between two rotatable members and is the sum of thenumber of degrees by which one rotatable member may be rotated relativeto the other rotatable member in both the positive and negativedirections.

The phrase “means for” when used in a claim is intended to and should beinterpreted to embrace the corresponding structures and materials thathave been described and their equivalents. Similarly, the phrase “stepfor” when used in a claim embraces the corresponding acts that have beendescribed and their equivalents. The absence of these phrases means thatthe claim is not intended to and should not be interpreted to be limitedto any of the corresponding structures, materials, or acts or to theirequivalents.

Nothing that has been stated or illustrated is intended or should beinterpreted to cause a dedication of any component, step, feature,object, benefit, advantage, or equivalent to the public, regardless ofwhether it is recited in the claims.

The scope of protection is limited solely by the claims that now follow.That scope is intended and should be interpreted to be as broad as isconsistent with the ordinary meaning of the language that is used in theclaims when interpreted in light of this specification and theprosecution history that follows and to encompass all structural andfunctional equivalents.

1. A sensor for simultaneously sensing the angular position of a firstrotatable member relative to a frame of reference and the angularposition of a second rotatable member relative to the first rotatablemember up to a maximum displacement angle, the sensor comprising: afirst disk having a first annular pattern which alternates between twodifferent levels of optical transparency; a first coupling configured tocouple the first disk to the first rotatable member; a second diskhaving a second annular pattern which alternates between two differentlevels of optical transparency and which has substantially the sameradius as the first annular pattern, the second disk being configuredsuch that the first and the second annular patterns may independentlyrotate about substantially the same axis while remaining substantiallyadjacent to one another; a second coupling configured to couple thesecond disk to the second rotatable member; an optical sensing arrayconfigured to provide signals indicative of an annular illuminationpattern which is cast upon the optical sensing array and which has alength which is not substantially greater than the width of the annularillumination pattern cast upon the optical sensing array; a lightingsystem configured to direct light toward the first and the second diskswith an orientation that causes an annular illumination pattern to becast upon the optical sensing array which corresponds to a composite ofa portion of the first and the second annular patterns and which spansan angle that is less than or equal to the maximum displacement angle;and a signal processing system configured to generate an output that isindicative of both of the angular positions based on the signals fromthe optical sensing array.
 2. The sensor of claim 1 further comprising adata storage system containing calibration data, and wherein the signalprocessing system is configured to utilize the calibration data whengenerating the output.
 3. The sensor of claim 2 wherein: the opticalsensing array has a plurality of pixel elements, each of which providesa signal indicative of a level of illumination which is cast upon thepixel element; and the calibration data includes data indicative of thelevel of the signal from each pixel element when a known portion of thepixel element is illuminated.
 4. The sensor of claim 3 wherein the knownportion is approximately half of the pixel element.
 5. The sensor ofclaim 2 wherein: the optical sensing array has a plurality of pixelelements; and the calibration data includes data indicative of thelocation of each pixel element.
 6. The sensor of claim 2 wherein: theoptical sensing array has a plurality of pixel elements, each of whichprovides a signal indicative of the level of illumination which is castupon the pixel element; and the calibration data includes dataindicative of the rate at which the signal that is provided by eachpixel element changes as a function of changes in the level ofillumination which is cast upon each pixel element.
 7. The sensor ofclaim 1 further comprising a data storage system containing calibrationdata that includes data indicative of a location error in each of thetransitions between the two different levels of optical transparency ineach of the annular patterns, and wherein the signal processing systemis configured to utilize the calibration data when generating theoutput.
 8. The sensor of claim 1 wherein each of the disks are formed bya stamping process.
 9. The sensor of claim 1 wherein each of the diskshas an integral race configured to support ball bearings.
 10. The sensorof claim 1 wherein the lighting system includes a light source and areflecting system configured to focus and collimate light from the lightsource onto the first and the second disks.
 11. The sensor of claim 10wherein the light source and the optical sensor array are adjacent tothe same side of one of the disks.
 12. The sensor of claim 11 whereinthe light source and the optical sensor are mounted to a common circuitboard.